Biodegradable polymers which are decomposed in the natural environment are attracting attention and being studied worldwide for the purpose of global environmental protection. As the biodegradable polymers, there are known polyhydroxybutyrate, polycaprolactone, aliphatic polyesters and polylactic acid. They can be melt molded and are expected to be used as general-purpose polymers. Since lactic acid or lactide which is the raw material of polylactic acid out of these can be manufactured from natural products, use of the polylactic acid not as just a biodegradable polymer but as a general-purpose polymer prepared by taking global environment into consideration is now under study. Although the polylactic acid has high transparency and toughness, it is easily hydrolyzed in the presence of water and decomposed without contaminating the environment after it is scrapped. Therefore, it is expected to be used as a general-purpose polymer having a small environmental load.
Since the melting point of the polylactic acid is in the range of 150 to 170° C., when it is used as a apparel fiber, the temperature for ironing the fiber is limited to a low temperature. When it is used as an industrial fiber, it is not suitable for use as a rubber material or resin coated dishcloth which is exposed to a high temperature of about 150° C. as a production temperature.
Meanwhile, it is known that when poly(L-lactic acid) which is composed of only an L-lactic acid unit (may be referred to as “PLLA” hereinafter) and poly(D-lactic acid) which is composed of only a D-lactic acid unit (may be referred to as “PDLA” hereinafter) are mixed together in a solution or molten state, stereocomplex polylactic acid is formed (non-patent document 1). It is also known that this stereocomplex polylactic acid has a higher melting point than those of PLLA and PDLA and shows high crystallinity. Various studies on fibers made of the stereocomplex polylactic acid are also under way.
For example, patent document 1 discloses a stereocomplex polylactic acid fiber obtained by melt spinning a composition containing equimolar amounts of poly(L-lactic acid) and poly(D-lactic acid). However, the stereocomplex polylactic acid fiber is unsatisfactory in terms of heat resistance and cannot be put to practical use.
Non-patent document 2 discloses that a stereocomplex polylactic acid fiber is obtained by melt spinning. This document teaches that the stereocomplex fiber is obtained by heating unstretched yarn obtained by melt spinning a molten blend of poly(L-lactic acid) and poly(D-lactic acid). However, as molecular orientation in the inside of the fiber is alleviated at the time of heating, the strength of the obtained fiber is only 2.3 cN/dTex.
In the conventional stereocomplex forming method, amorphous unstretched yarn obtained by spinning a blend of poly(L-lactic acid) and poly(D-lactic acid) is stretched and heated. That is, in the prior art, based on the idea that it is efficient to heat stereocomplex at a temperature equal to or higher than the melting point of a poly(L-lactic acid) or poly(D-lactic acid) homocrystal in order to fully grow the stereocomplex, the heat treatment is mainly carried out at a temperature higher than the melting point of the homocrystal. It has been certain that this high-temperature heat treatment has been effective for the formation of the stereocomplex. However, when the heat treatment is carried out at a high temperature, the partial melting of the yarn occurs, whereby the yarn becomes rough and hard, or its strength lowers.
To cope with this problem, patent document 2 proposes a method of forming stereocomplex from molten polylactic acid on the line of spun yarn. For example, it is proposed that the partial melting of the yarn should be improved by carrying out spinning at a high rate of 4,000 m/min and stretching crystallized unstretched yarn having a stereo crystallization ratio of 10 to 35% when measured by wide-angle X-ray diffraction (XRD) to 1.4 to 2.3 times. However, to carry out this method, a spinning rate of 3,000 m/min is not satisfactory and a special spinning apparatus for spinning at a rate of not less than 5,000 m/min is required. Therefore, there are problems to be solved for carrying out this method industrially. As for the evaluation of heat resistance in this proposal, an iron heated at 170° C. is applied to a tubular knit fabric of the fiber to see a significant change such as a rupture or roughening and hardening of the knitted fabric, but the shrinkage of apparel made of apparel fibers is not studied at all. Thus, heat resistance is not studied completely. A technology for manufacturing a fiber having a high stereo crystallization ratio and excellent strength and heat shrinkage resistance from unstretched yarn having a stereo crystallization ratio of 0% is not accomplished yet.
Patent document 3 proposes a fiber having two peaks derived from a polylactic acid homocrystal and a stereocomplex crystal at 190° C. or higher and a heat resistance of 200° C., which is obtained by stretching unstretched yarn obtained by melt spinning at a spinning draft of not less than 50 and a take-up rate of not less than 300 m/min to 2.8 times after the unstretched yarn is wound up or without winding it up and by heating it at 120 to 180° C.
Meanwhile, patent document 4 proposes that a phosphate metal salt is contained in polylactic acid capable forming stereocomplex as a crystal nucleating agent to improve the heat resistance and impact resistance of a molded article.    (patent document 1) JP-A 63-241024    (Patent Document 2) JP-A 2003-293220    (Patent Document 3) JP-A 2005-23512    (patent document 4) JP-A 2003-192884    (non-patent document 1) Macromolecules, 24, 5651 (1991)    (non-patent document 2) Seni Gakkai Preprints (1989)